At a Boiled Water Reactor (BWR) power plant, to measure neutron flux in a nuclear reactor, Local Power Range Monitors (LPRMs) are provided in the reactor. Fissile material provided at an electrode in the LPRMs fissions and releases ionized atoms as a result of being irradiated with neutrons. Neutron flux is obtained by measuring the ionized atoms.
However, the LPRMs are placed in the reactor constantly and are difficult to replace, and the sensitivity of the LPRMs declines with time because of consumption of the fissile material. Therefore, the sensitivity of the LPRMs needs to be calibrated at different times to measure neutron flux in the reactor precisely.
At a Pressurized Water Reactor (PWR) power plant, to measure neutron flux in a nuclear reactor, fixed neutron sensors are provided at the outer periphery of the reactor. But the sensitivity of these fixed neutron sensors also declines with time and needs to be calibrated at different times.
To calibrate the sensitivity of the LPRMs of the BWR and fixed neutron sensors of the PWR, a traveling reactor power monitoring system is provided in the nuclear power plant. Generally, the traveling reactor power monitoring system in the BWR is referred to as a Traversing In-core Probe (TIP) monitoring system.
This TIP monitoring system moves a traveling probe, referred to as a TIP, in guide tubes provided in the reactor, and the TIP measures neutron flux in the proximity of the LPRMs while moving. By using measured neutron flux, the sensitivity of the LPRMs is calibrated.
At a PWR power plant, the traveling reactor power monitoring system referred to as a Flux Mapping System moves a traveling probe (TIP) in the reactor, and the sensitivity of the fixed neutron sensors is calibrated by using measured neutron flux or gamma rays measured by the traveling probe.
The traveling probe is attached to an edge of a probe cable, and the traveling probe moves in the guide tubes by feeding the probe cable from a storage reel and spooling the probe cable onto the storage reel by rotating a motor. This traveling probe measures radioactivity such as neutrons and gamma rays as the reactor power during moving in the reactor core.
At this point, to reduce friction while the traveling probe and the probe cable move in the guide tube, the inside of the guide tubes can be coated with a lubricant.
When the drive torque that is a torque necessary for moving the traveling probe and the probe cable exceeds an appropriate value, there is a possibility of damaging the guide tube, the traveling probe, and the probe cable by excessive friction. Therefore, the friction inside of the guide tubes can be monitored by measuring the drive torque.
Japanese Patent Laid-open Publication No. 2002-71483 discloses a traveling reactor power monitoring system having a torque sensor attached to the motor shaft. That torque sensor measures the drive torque automatically.
The above mentioned traveling reactor power monitoring system compares the drive torque and a predefined upper threshold during the neutron measuring operation, and generates an alarm when the drive torque exceeds the upper threshold.
However, due to an inrush current or static frictional force, the drive torque at a time of starting driving the motor may be larger than the drive torque at driving the motor. As a result, despite an acceptable friction condition, the existing traveling reactor power monitoring system may generate an alarm when the drive torque at a time of starting driving the motor exceeds the upper threshold.